JP 2009-185765A discloses one exemplary configuration of an electronic control apparatus, which controls a vehicle travel speed to a constant speed. This conventional apparatus stores as a reference vehicle speed a vehicle speed, when an accelerator operation angle and a vehicle speed are both constant for a predetermined period. When the vehicle speed changes from the reference vehicle speed by more than a predetermined value in spite of the accelerator operation angle being maintained at the same operation angle, the vehicle speed is controlled to return to the reference vehicle speed.
When a vehicle travels, the vehicle receives traveling resistances such as air resistance, rolling resistance and slope resistance. The air resistance and the rolling resistance are variable with a shape, a weight and the like of an individual vehicle. The slope resistance is variable with a road characteristic such as an inclination of a road. For this reason, when the weight of the vehicle increases or the vehicle enters an uphill slope from a level road, the traveling resistance increases and impedes acceleration of the vehicle, which a driver desires. In this respect, the conventional apparatus controls the vehicle speed to return to the reference vehicle speed when the vehicle speed deviates from the reference speed by the predetermined value. This control however only returns the vehicle speed to the reference vehicle speed, which was stored when the accelerator operation angle was constant for the predetermined period. That is, the conventional apparatus cannot increase the vehicle speed to be higher than the reference vehicle speed even when the driver desires a higher vehicle speed. As a result, it is not possible to attain the acceleration, which the driver desires.